This section provides background information related to the present disclosure that is not necessarily prior art.
Electric-based vehicles or EVs (e.g., hybrid electric vehicles (HEV), battery electric vehicles (BEV), plug-in HEVs, and extended-range electric vehicles (EREV)) require efficient, low-cost, and safe energy storage systems with high energy density and high power capability. Lithium ion batteries can be used as a power source in many applications ranging from vehicles to portable electronics such as laptop computers, cellular phones, and so on. The EVs powered by the current lithium cobalt or lithium-iron phosphate batteries often have a driving range of less than 100 miles (160 km) per charge, while longer driving ranges would be desirable.
A battery based on Li—S chemistry offers an attractive technology that meets the two most pressing issues for electric-based transportation, the needs for low cost and high specific density. Li—S battery technology has been the subject of intensive research and development both in academia and in industry due to its high theoretical specific energy of 2600 Wh/kg as well as the low cost of sulfur. The theoretical capacity of sulfur via two-electron reduction (S+2Li++2e−⇄Li2S), is 1672 mAh/g (elemental sulfur is reduced to S−2 anion). The discharge process starts from a crown S8 molecule and proceeds though reduction to higher-order polysulfide anions (Li2S8, Li2S6) at a high voltage plateau (2.3-2.4 V), followed by further reduction to lower-order polysulfides (Li2S4, Li2S2) at a low voltage plateau (2.1 V), and terminates with the Li2S product. During the charge process, Li2S is oxidized back to S8 through the intermediate polysulfide anions Sx. The Sx polysulfides generated at the cathode are soluble in the electrolyte and can migrate to the anode where they react with the lithium electrode in a parasitic fashion to generate lower-order polysulfides, which diffuse back to the cathode and regenerate the higher forms of polysulfide. Y. V. Mikhaylik & J. R. Akridge, “Polysulfide Shuttle Study in the Li/S Battery System,” J. Electrochem. Soc., 151, A1969-A1976 (2004) and J. R. Akridge, Y. V. Mikhaylik & N. White, “Li/S fundamental chemistry and application to high-performance rechargeable batteries,” Solid State Ionics, 175, 243-245 (2005) describe this shuttle effect, which leads to decreased sulfur utilization, self-discharge, poor ability to repeatedly cycle through oxidation and reduction, and reduced columbic efficiency of the battery. The insulating nature of S and Li2S results in poor electrode rechargeablity and limited rate capability. In addition, an 80% volume expansion takes place during discharge. Overall, these factors preclude the commercialization of Li—S batteries for EVs.
The theoretical energy density of a Si—S battery is comparable to that of a Li—S battery. However, because a Si—S battery operates on the same sulfur chemistry, it suffers from the same problem of polysulfide diffusion to the anode.
To circumvent these obstacles, extensive effort has been devoted to the development of better sulfur cathodes, which has mainly relied on infiltration or in situ growth of sulfur into or onto conductive scaffolds, such as conductive polymers (e.g., polythiophene, polypyrrole, and polyaniline) and porous carbons (e.g., active carbons, mesoporous carbons, hollow carbon spheres, carbon fibers, and graphene). It has been found that, generally, the incorporation of sulfur within conductive polymers results in sulfur/polymer cathodes with improved capacity and cycling stability. The sulfur and the polymer may be crosslinked, leading to electrodes with further improved cycling life. Compared with polymeric scaffolds, carbon scaffolds offer many advantages, such as better stability and conductivity, low cost, and controllable pore structure, which make them more attractive candidates for sulfur cathodes. Polymers (e.g., poly(ethylene oxide) and poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate)) may be coated on the carbon/sulfur composites to further improve the cycling life and coulomb efficiency. However, despite extensive efforts being made, current sulfur cathodes still fail to meet the requirement of high-performance Li/S batteries. Current sulfur cathodes do not sufficiently retard polysulfide migration to be able to prolong cathode cycling life. During discharge of current sulfur/carbon cathodes, the cyclic S8 molecules are converted to polysulfides (Li2Sn, 2<n<8). Driven by the concentration gradient, the polysulfides dissolved in the electrolyte unavoidably diffuse away from the cathodes, causing fast capacity fading with poor cycling life. Nevertheless, a functioning cathode also requires effective lithium ion transport between the electrolyte and the electrodes. Because electrolyte molecules, lithium ions, and the polysulfides exhibit comparable diffusion coefficients, carbon materials that are able to retard the outward polysulfide diffusion will also retard the transport of electrolyte and lithium ions, resulting in poor rate performance or even dysfunction of the cathode. This fundamental dilemma has until now prevented the art from realizing the great potential of Li/S batteries.
Silicon sulfur batteries also use a sulfur cathode and, therefore, are subject to the same issues of polysulfide migration.